Directional microphones are frequently used to accentuate a useful acoustic signal in an environment filled with interference noise. For example a speech signal is to be accentuated against the ambient noise in a hearing device with a directional microphone. In such cases directional microphones in hearing devices have for many years been among the established methods of reducing interference noise and have demonstrably led to improving the recognizability of speech in hearing situations in which the useful signal and the interference signals are entering the device from different directions in the room.
When a directional microphone is incorporated into the device, two different types are widely used:
a) Gradient Microphones:
These possess two sound entry points which lead to different sides of one and the same membrane of the gradient microphone. If sound arrives simultaneously at both sound entry points, the forces thus created on the membrane cancel each other out. The output signal in this case is equal to zero. The following general points apply: Sound which enters at right angles to the connecting line of the sound entry openings is extinguished. The disadvantage of gradient microphones is that they are barely able to be adjusted for interference sources which do not remain in a fixed location in relation to the microphones.
b) Electrically Connected Omnidirectional Microphones:
Omnidirectional microphones have one sound entry opening and ideally accept sound from all directions equally. A directional effect can be created by electrical connection of at least two omnidirectional microphones. To do this one directional microphone signal is delayed and subtracted from the microphone signal of a second omnidirectional microphone. Precisely as with the gradient microphone, with the microphone system just described, by a particular arrangement of the sound entry openings and adjusting the delay time a direction can be defined for which the incident sound from this direction is extinguished. A first-order direction effect can be created with two omnidirectional microphones connected electrically to each other. With an electrical connection or more than two omnidirectional microphones can directional arrangements of higher orders can also be created.
The invention relates to directional microphones comprising at least two omnidirectional microphones connected electrically to each other and which, by adjusting the delay time(s) provide the opportunity for simple alteration of the directional characteristic during operation of the directional microphone.
Directional microphones which comprise a number of omnidirectional microphones stand out from a single omnidirectional microphone not because a specific direction is especially well received, but because one (or more) direction(s) is (are) suppressed in relation to the non-directed (omnidirectional) microphone. This is illustrated graphically in what are known as polar diagrams. In these diagrams the attenuation in dB is mostly plotted for an acoustic input signal against the angle of incidence. A position with very high attenuation is referred to as a notch in such a diagram. Depending on the position and number of the notches, different detector characteristics are produced (kidney-shaped characteristic, figure-of-eight-shaped characteristic etc).
With a static directional microphone a specific directional characteristic is fixed by selecting a specific delay time or specific delay times. With a directional microphone constructed from two omnidirectional microphones, the maximum directional effect achievable with the directional microphone, expressed by the so-called directivity index (DI), is obtained when a hypercardioid characteristic is set. This means that, for a directional microphone which is subjected to diffuse sound entering it in a free field evenly from all directions, the output signal has the lowest energy or power at this setting. Static directional microphones in hearing devices are frequently adjusted to such a setting.
A static directional characteristic of a directional microphone optimized in the free field is further worsened when a directional microphone is used in a hearing device if the hearing device is worn on a user's head by the influence of the head, since the head changes both the amplitude and also the phase of the signals picked up by the microphone. This also worsens the maximum directional effect that can be achieved by the directional microphone. From a hypercardioid set in the free field with maximum DI for example another directional characteristic will arise which has its notch at another angle and will thus no longer possess an optimum DI.
Compensating for the negative influence of the head on the optimum directional effect by not optimizing the directional effect in the free field but on an artificial head created for test purposes, e.g. the KEMAR, and thereby at least reducing the negative head effects, is known. The problem now however is that the influence of the head and the pinna can be individually quite different and the improvements achieved on an average artificial head are not optimized for the relevant individual electrophysiological situations.
An adaptive directional microphone with a number of microphones electrically connected to each other is known from US 2001/0028718 A1, in which the directional effect is continuously adapted during ongoing operation of the directional microphone to different hearing situations. The known directional microphone comprises means for determining the energy of the directional microphone signal created by the directional microphone, through which interference signals from different incident directions can be suppressed very quickly in the microphone system as a result of very short adaptation times. However the adaptive directional microphone does not provide any advantage worth mentioning over a static directional microphone in situations with predominantly diffuse, i.e. non-directed interference noise (e.g. a cafeteria).
Until now directional microphones have been operated either as static directional microphones in which the delay time(s) is (are) set once and then retained, or as adaptive directional microphones which react quickly to changing environmental situations and adaptively suppress interference noise. The time constants used with adaptive directional microphones are usually less than a second.